Let me share what I know about splitting accounts in QuickBooks Online (QBO).
If the two self accounting companies are reported under one payroll and sales tax, then you can use the class and location tracking feature in QBO. This helps track of your income and expenses using class and location.
First, you'll want to turn on this feature in QBO:
Go to Settings.
Click Account and Settings.
Select Advanced from the left menu.
On the Categories section, select the pencil icon.
Choose the category you want to enable, then select the settings for that category.